The invention relates to an exhaust-gas cleaning system for cleaning the exhaust gas from a combustion source so as to remove at least nitrogen oxides which are contained therein having an ammonia-generation catalytic converter for generating ammonia using constituents of at least some of the exhaust gas emitted from the combustion source during ammonia-generation operating phases, and a nitrogen oxide reduction catalytic converter, which is connected downstream of the ammonia-generation catalytic converter, for reducing nitrogen oxides which are contained in the exhaust gas emitted from the combustion source using the ammonia generated as the reducing agent.
Exhaust-gas cleaning systems of this type are used in particular for exhaust-gas cleaning in motor vehicle internal-combustion engines and are described, for example, in publications EP 0 802 315 A2 and WO 97/17532 A1. They include a nitrogen oxide reduction catalytic converter for the selective catalytic reduction of nitrogen oxides which are contained in the exhaust gas emitted from the combustion source using ammonia as the reducing agent, referred to for short as the SCR process. In order that it is not necessary to hold a stock of ammonia or a precursor in a tank, an ammonia-generation catalytic converter is connected upstream of the nitrogen oxide reduction catalytic converter, the ammonia-generation catalytic converter generating the ammonia which is required using constituents of at least some of the exhaust gas which is emitted from the combustion source during corresponding ammonia-generation operating phases, specifically by means of a synthesis reaction of hydrogen and nitrogen monoxide. In these ammonia-generation operating phases, a rich air ratio is set for the exhaust gas which is fed to the ammonia-generation catalytic converter, in order that sufficient hydrogen be available. In this context, the terms rich and lean air ratio, also known as the lambda value, are understood as meaning, as is customary, a composition of the exhaust gas or of the associated fuel mixture burnt in the combustion source which deviates from the stoichiometric composition towards being fuel-rich or oxygen-rich, respectively. In this context, if only for fuel consumption reasons it is desired for the combustion source to be operated as much as possible in lean-burn mode and as little as possible in rich-burn mode, for example as a result of prolonged lean-burn operating phases alternating with brief rich-burn operating phases or, in the case of a multicylnder internal-combustion engine, only some of the cylinders, and preferably likewise only from time to time, being operated in rich-burn mode, whereas the other cylinders are continuously operated in lean-burn mode.
The ammonia-generation catalytic converter used is usually a three-way catalytic converter which contains as the catalyst material, by way of example, Pt and/or Rh supported on γ—Al2O3, which is suitable for catalysing the synthesis reaction of hydrogen and nitrogen monoxide to form ammonia. However, it has been found that without further measures the selectivity for effective ammonia formation by this synthesis reaction is only present at a sufficiently high temperature of the order of magnitude of above approximately 250° C. to 300° C. This is primarily attributable to the fact that the selectivity of this catalytic ammonia synthesis reaction only rises to a level which can be used in practice when this temperature is exceeded.
The invention is based on the technical problem of providing an exhaust-gas cleaning system of the type mentioned in the introduction in which ammonia can be synthesized in significant quantities even at relatively low temperatures of below approximately 250° C. to 300° C. and is available as a reducing agent for nitrogen oxide reduction at such temperatures.
The invention solves this problem by providing an exhaust-gas cleaning system wherein a plasma generator is connected upstream of the ammonia-generation catalytic converter for generating reactive particles using plasma technology from constituents of the exhaust gas which is fed to the ammonia-generation catalytic converter during the ammonia-generation operating phases, which reactive particles assist the ammonia-generation reaction in the ammonia-generation catalytic converter. This system characteristically contains a plasma generator connected upstream of the ammonia-generation catalytic converter. This generator, at least from time to time during the ammonia-generation operating phases, generates a plasma through which the exhaust gas which is then fed to the ammonia-generation catalytic converter is passed. The plasma-generation parameters are set in such a way that reactive particles such as H, OH and/or O2H free radicals are formed from constituents of the exhaust gas passed through, which free radicals promote the ammonia-generation reaction in the ammonia-generation catalytic converter. It is thus possible especially, ever in the low temperature range, in which the ammonia synthesis reaction from the exhaust-gas constituents does not proceed effectively without further assistants, for ammonia to be generated internally in significant amounts, which is then available for the nitrogen oxide reduction. External metering of the ammonia or a precursor in these periods with a relatively low ammonia-generation catalytic converter temperature can therefore generally be dispensed with without having to forego an effective, ammonia-based nitrogen oxide reduction.
In an exhaust-gas cleaning system which is refined means for detecting the ammonia-generation catalytic converter temperature and a plasma control unit are provided, in such a manner that during the ammonia-generation operating phases the plasma for the generation of reactive particles is provided precisely in those periods in which the temperature of the ammonia-generation catalytic converter is below a predeterminable temperature threshold. This threshold is expediently selected in such a way that at temperatures above the threshold effective ammonia synthesis is effected in the ammonia-generation catalytic converter even without the reactive particles generated using plasma technology. In a further, preferred configuration of this measure, the corresponding plasma control unit is designed for a temperature threshold of between 200° C. and 300° C., preferably for a threshold of approximately 250° C. It has been found that effective plasma-assisted ammonia synthesis can be effected below this temperature range and effective ammonia synthesis can be effected even without additional plasma activation above this temperature range.